Nailing Northolt

Nailing Northolt

The organically-named Evergreen 3 is the interesting project to transform the route f:om Birmingham Moor Street to London Marylebone to allow 100mph running. The entire scheme is being sponsored by Chiltern Railways, particularly topical in these times of discussions on devolution and vertical integration.

The first phase, which is nearing completion, includes remodeling three junctions, at Neasden, Northolt and Aynho. Work is being undertaken at Princes Risborough to restore the original up through line. There will be a new turn back loop at Gerrards Cross, and a new, more direct down line is being built at Northolt Junction. The launch service on this improved line is scheduled for Monday 5 September.

The rail engineer visited the works at Northolt Junction recently, and reported on the bridge replacement there in issue 77 of the rail engineer (March 2011). However, all these improvements have needed extensive earthwork stabilisation to freshly-widened cuttings and embankments, and to do this the BAM Nuttall delivery team brought in geotechnical specialists BAM Ritchies.

Gerrards Cross

Starting on site at the beginning of October 2010, the BAM Ritchie’s team stabilised the steepened slopes at Gerrards Cross. 4m was trimmed from the bottom of the batters and the slope re-profiled to 65°, helping to provide working space for the track realignment and new drainage installed in the area. During three weekend line possessions, the team installed 302 soil nails, mostly 14m long, 32mm diameter with a hollow stem, using road/rail excavator mounted drilling equipment. The nails were simultaneously grouted while rock fall mesh with combined erosion matting helped stabilise the re-profiled slopes.

Even more soil nails were needed at Northolt where 3,900 nails were installed, the bulk of them being self‑ drilling 32mm diameter nails 14m long. These stabilised the slopes of an extended existing embankment and also boosted its load bearing capacity.

Northolt

The site at Northolt is split into four quadrants. Over 7,000m2 of slope facing, consisting of rock fall mesh with combined erosion matting, was placed in under four weeks to stabilise the regarded northern sector. This strengthened the embankment allowing the existing Up Main to be slewed across.

In the southern sector, nails were installed through new gabions with extra fill plated behind them to widen the embankment. This allowed the new Down Main to be constructed adjacent to the slewed Up Main, thus enabling the enhanced line speed through re-alignment. The nails were drilled through the extra fill and into the existing material.

Ground conditions were well suited to self-drilling nails according to Andrew O'Donovan, BAM Ritchie’s' Contracts Manager. ‘It’s all London Clay other than the fill material which is pretty good for our installation. It drills well and we get a good return of material when grout flushing; he stated.
Most nails were installed using 6m masts so minimal bar additions were required to reach the required depth of 14m. However, where nails were positioned closer than 6m to the boundary, a sectional mast was used. By quickly altering the length of the mast the section length of each nail could be maximised.

At the peak of the work, over the Easter weekend, five drilling rigs were in use simultaneously.

King Posts

BAM Ritchies also drilled 163 boreholes along the length of a king post wall at Northolt. Using a drilling rig equipped with 450mm diameter, 1m long augers, the team bored holes 6-8m deep and installed 254 x 254 x 107mm steel universal columns to help form the king post retaining wall.
41 similar columns were used six miles further towards Marylebone, at Neadsen Junction. Another king post wall was constructed during three 12- hour possessions to retain the higher level LUL relay room and fencing whilst the adjacent Up Harrow line was constructed.

Article courtesy of The Rail Engineer magazine August 2011